Microlithography (also called photolithography or simply lithography) is a technology for the fabrication of integrated circuits, liquid crystal displays and other microstructured devices. More particularly, the process of microlithography, in conjunction with the process of etching, is used to pattern features in thin film stacks that have been formed on a substrate, for example a silicon wafer. At each layer of the fabrication, the wafer is first coated with a photoresist which is a material that is sensitive to radiation, such as deep ultraviolet (DUV) light. Next, the wafer with the photoresist on top is exposed to projection light in a projection exposure apparatus. The apparatus projects a mask containing a pattern onto the photoresist so that the latter is only exposed at certain locations which are determined by the mask pattern. After the exposure the photoresist is developed to produce an image corresponding to the mask pattern. Then an etch process transfers the pattern into the thin film stacks on the wafer. Finally, the photoresist is removed. Repetition of this process with different masks results in a multi-layered microstructured component.
A projection exposure apparatus typically includes an illumination system for illuminating the mask, a mask stage for a aligning the mask, a projection objective and a wafer alignment stage for aligning the wafer coated with the photoresist. The illumination system illuminates a field on the mask that may have the shape of an elongated rectangular slit, for example.
In current projection exposure apparatus a distinction can be made between two different types of apparatus. In one type each target portion on the wafer is irradiated by exposing the entire mask pattern onto the target portion in one go. Such an apparatus is commonly referred to as a wafer stepper. In the other type of apparatus, which is commonly referred to as a step-and-scan apparatus or scanner, each target portion is irradiated by progressively scanning the mask pattern under the projection beam in a given reference direction while synchronously scanning the substrate table parallel or anti-parallel to this direction. The ratio of the velocity of the wafer and the velocity of the mask is equal to the magnification of the projection objective, which is usually smaller than 1, for example 1:4.
It is to be understood that the term “mask” (or reticle) is to be interpreted broadly as a patterning device. Commonly used masks contain transparent or reflective patterns and may be of the binary, alternating phase-shift, attenuated phase-shift or various hybrid mask type, for example. However, there are also active masks, e.g. masks realized as a programmable mirror array. An example of such a device is a matrix-addressable surface having a viscoelastic control layer and a reflective surface. More information on such mirror arrays can be gleaned, for example, from U.S. Pat. Nos. 5,296,891, 5,523,193, 6,285,488 B1, 6,515,257 B1 and WO 2005/096098 A2. Also programmable LCD arrays may be used as active masks, as is described in U.S. Pat. No. 5,229,872. For the sake of simplicity, the rest of this text may specifically relate to apparatus including a mask and a mask stage. However, the general principles discussed in such apparatus should be seen in the broader context of the patterning device as hereabove set forth.
As the technology for manufacturing microstructured devices advances, there are ever increasing demands also on the illumination system. Ideally, the illumination system illuminates each point of the illuminated field on the mask with projection light having a well defined irradiance and angular distribution. The term angular distribution describes how the total light energy of a light bundle, which converges towards a particular point in the mask plane, is distributed among the various directions along which the rays constituting the light bundle propagate. Frequently the angular distribution in the mask plane is simply referred to as illumination setting.
The angular distribution of the projection light impinging on the mask is usually adapted to the kind of pattern to be projected onto the photoresist. For example, relatively large sized features may involve a different angular distribution than small sized features. The most commonly used angular distributions of projection light are referred to as conventional, annular, dipole and quadrupole illumination settings. These terms refer to the irradiance distribution in a pupil surface of the illumination system. With an annular illumination setting, for example, only an annular region is illuminated in the pupil surface. Thus there is only a small range of angles present in the angular distribution of the projection light, and thus all light rays impinge obliquely with similar angles onto the mask.
Different approaches are known in the art to modify the angular distribution of the projection light in the mask plane so as to achieve the desired illumination setting. In the simplest case an aperture stop (diaphragm) including one or more apertures is positioned in a pupil surface of the illumination system. Since locations in a pupil surface translate into angles in a Fourier related field plane such as the mask plane, the size, shape and location of the aperture(s) in the pupil surface determines the angular distributions in the mask plane. However, any change of the illumination setting involves a replacement of the stop. This can make it difficult to finally adjust the illumination setting, because this would involve a very large number of stops that have aperture(s) with slightly different sizes, shapes or locations. Apart from that, such a stop can absorb a very significant amount of light. This can reduce the throughput of the entire projection exposure apparatus.
Many common illumination systems therefore include adjustable elements that make it possible, at least to a certain extent, to continuously vary the illumination of the pupil surface. Conventionally, a zoom axicon system including a zoom objective and a pair of axicon elements are used for this purpose. An axicon element is a refractive lens that has a conical surface on one side and is usually plane on the opposite side. By providing a pair of such elements, one having a convex conical surface and the other a complementary concave conical surface, it is possible to radially shift light energy. The shift is a function of the distance between the axicon elements. The zoom objective makes it possible to alter the size of the illuminated area in the pupil surface.
However, with such a zoom axicon system only conventional and annular illumination settings can typically be produced. For other illumination settings, for example dipole or quadrupole illumination settings, additional stops or optical raster elements are often involved. An optical raster element produces, for each point on its surface, an angular distribution which corresponds in the far field to certain illuminated areas. Often such optical raster elements are realized as diffractive optical elements, and in particular as computer generated holograms (CGH). By positioning such an element in front of the pupil surface, optionally with an additional condenser lens in between, it is possible to produce almost any arbitrary intensity distribution in the pupil surface. An additional zoom-axicon system may be used to vary, to a limited extent, the illumination distribution produced by the optical raster element in the pupil surface.
However, the zoom axicon system typically provides only limited adjustability of the illumination setting. For example, it is generally not possible to dislocate only one of the four poles of a quadrupole illumination setting along an arbitrary direction. To this end another optical raster element often has to be used that is specifically designed for this particular intensity distribution in the pupil surface. The design, production and shipping of such optical raster elements can be a time consuming and costly process, and thus there is often little flexibility to adapt the light intensity distribution in the pupil surface to the needs of the operator of the projection exposure apparatus.
For increasing the flexibility in producing different angular distribution in the mask plane, it has been proposed to use mirror arrays that illuminate the pupil surface.
In EP 1 262 836 A1 such a mirror array is realized as a micro-electromechanical system (MEMS) including more than 1000 microscopic mirrors. Similar illumination systems are known from other patent documents, such as US 2006/0087634 A1 and U.S. Pat. No. 7,061,582 B2.
WO 2005/026843 A2 discloses an illumination system in which a diffractive optical element is arranged in the beam path between a mirror array and a pupil surface of the illumination system. An optical integrator such as a fly's eye lens or a quartz rod is arranged between the mirror array and the mask. The optical integrator ensures a homogenous illumination of the mask and also defines at least approximately the geometry of the illuminated field.